1. Field of the Invention
The present invention relates to a method and an apparatus for polishing a workpiece, and more particularly to a method and an apparatus for polishing a workpiece such as a semiconductor wafer, a glass substrate, or a liquid crystal panel which is required to be highly cleaned.
2. Description of the Related Art
As semiconductor devices become more highly integrated in recent years, circuit interconnections become finer and the distances between those interconnections also become smaller. Photolithographic processes for producing interconnections that are 0.5 xcexcm wide or smaller, particularly, require a flat image-focusing plane for the stepper because the depth between focal points is small. If a dust particle whose size is greater than the distances between the interconnections is present on a semiconductor substrate, then an undesirable short circuit tends to occur between interconnections through the dust particle.
Therefore, it is important that the workpiece processing must produce a flat and clean workpiece. These processing requirements apply equally to other workpiece materials, such as glass substrates for photo-masking or liquid crystal display panels.
One conventional polishing apparatus is shown in FIG. 9 of the accompanying drawings. As shown in FIG. 9, the conventional polishing apparatus includes a polishing unit 10, a loading/unloading unit 21, a feed robot 22, and two cleaning machines 23a, 23b. As shown in FIG. 10 of the accompanying drawings, the polishing unit 10 comprises a turntable 12 with a polishing cloth 11 attached to an upper surface thereof, and a top ring 13 for holding a workpiece 1 such as a semiconductor wafer and pressing the workpiece 1 against the polishing cloth 11 on the turntable 12.
In operation, the workpiece 1 is supported on the lower surface of the top ring 13, and pressed by a lifting/lowering cylinder against the polishing cloth 11 on the turntable 12 which is being rotated. A polishing solution (abrasive solution) Q is supplied from a polishing solution nozzle 14 onto the polishing cloth 11 and retained by the polishing cloth 11. The lower surface of the workpiece 1 is polished by the polishing cloth 11 while the polishing solution Q is being present between the workpiece 1 and the polishing cloth 11.
The turntable 12 and the top ring 13 rotate at respective speeds that are independent of each other. The top ring 13 holds the workpiece 1 with its edges being spaced distances xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d from the center and the circumferential edge of the turntable 12. Thus, the entire lower surface of the workpiece 1 is uniformly polished at a high polishing rate. The workpiece 1 has a diameter xe2x80x9cdxe2x80x9d. The turntable 12 has a diameter xe2x80x9cDxe2x80x9d which is selected to be at least twice the diameter xe2x80x9cdxe2x80x9d of the workpiece 1, as indicated by the following equation:
D=2(d+a +b)
After having been polished, the workpiece 1 is cleaned in one or more cleaning processes and dried by the cleaning machines 23a, 23b, and then housed in a delivery cassette 24 of the loading/unloading unit 21. When the workpiece 1 is cleaned, it may be scrubbed by a brush of nylon, mohair or the like, or a sponge of PVA (polyvinyl alcohol).
In the conventional polishing apparatus, since the relative displacement between the turntable 12 and the top ring 13 is large, and the relative sliding speed between them is also large, the workpiece 1 can be polished efficiently to a flat finish. However, the polished surface of the workpiece 1 tends to have large surface roughness.
In order to produce a polished workpiece of better surface quality, consideration may be given to using two turntables which are operated by varying the surface roughnesses of the polishing cloths, rotational speeds and types of polishing solutions. However, as mentioned above, the diameter of the turntable is larger than twice that of the workpiece diameter, and each apparatus takes up a large floor space area which leads to higher facility costs. These problems becomes more ignorable as the semiconductor manufacturing industry seeks larger diameter substrates.
While it is possible to use one turntable to produce a superior surface quality by varying the type of polishing solution and lowering the rotational speed of the turntable, it is obvious that such an approach leads not only to a potential increase in the cost of polishing solution but also to inevitable lowering in the production efficiency due to a prolonged polishing operation.
In order to make the workpiece clean, there are some cases where scrubbing process is carried out after the workpiece 1 has been polished using the polishing solution Q. However, such scrubbing process fails to remove submicron particles from the polished surface of the workpiece 1, and is not effective enough to clean the polished workpiece 1 if remaining particles are bonded to the workpiece 1 by large bonding strength.
Further, the conventional polishing apparatus of the type described above has an advantage that the entire surface of the workpiece is polished uniformly, because the elasticity of the polishing cloth 11 moderates the effects of undulation and bowing in the workpiece. However, the workpiece such as a semiconductor wafer is susceptible to edge wear caused by excessive polishing around the peripheral edge of the wafer. Further, in order to polish semiconductor wafers with printed wiring patterns, it is required to obtain a polished surface having flatness of less than 1,000 angstrom by removing any micro-protrusions from uneven surface of the semiconductor wafer. However, the polishing cloth 11 is unable to meet this requirement because the elasticity of the polishing cloth allows the cloth itself to deform and the material from recessed regions as well as from protruding regions is removed.
It is therefore an object of the present invention to provide a method and an apparatus for polishing a workpiece such as a semiconductor wafer to a smooth flat finish with improved surface roughness, while effectively removing minute particles from the polished surface.
Another object of the present invention is to provide a compact polishing apparatus to produce a high degree of flatness of a workpiece such as a semiconductor wafer.
According to one aspect of the present invention, there is provided a method for polishing a workpiece, comprising: polishing a surface of the workpiece by pressing the workpiece against a polishing surface; and processing a polished surface of the workpiece by pressing the workpiece against a processing surface, the processing surface making relative translational motion relative to the workpiece.
According to another aspect of the present invention, there is also provided an apparatus for polishing a workpiece, comprising: a polishing unit for polishing a surface of the workpiece by pressing the surface of the workpiece against a polishing surface; a processing unit for processing a polished surface of the workpiece by pressing the workpiece against a processing surface, the processing surface making relative translational motion relative to the workpiece.
According to the present invention, the relative translational motion includes a relative motion of two surfaces of many patterns. The typical pattern is circular or circulative, i.e., repeating itself, and has a circular trace without respective rotational motion. However, it may include a respective rotation of a relatively large period of rotation compared to that of the circulative translation between the two surfaces. The trace of translation motion can be a linear translation pattern, a polygonal pattern or an elliptical pattern, but from the practical standpoint of polishing or processing efficiency and mechanical ease, a circular pattern would be optimum. In the circulative translation motion, all the regions of the workpiece are subjected to the same pattern.
In the present invention, a high removal ratio and a high flatness of the workpiece such as a semiconductor wafer is obtained in the polishing step, by subjecting the workpiece to a high speed material removal process with the polishing surface. In the processing step, the surface processing is carried out by a processing surface at a slow relative speed to attain a smooth surface of the workpiece, and also any micro-particles which may be adhered to the workpiece are removed. The surface of the workpiece is treated with a solution appropriate to the application. That is, in case that the processing step comprises a polishing step, abrasive particles are used while purified water or a suitable chemical solution is used in the processing step. In the processing step, abrasive particles are normally not used, and if they are used, a small amount of ultra-fine particles are used, and the pressing pressure of the workpiece against the processing surface is reduced compared to the polishing step.
Generally, a polishing apparatus of the circulatory translational motion type may have a processing surface such as a polishing cloth which is of relatively small dimensions. Then, the relative speed between the surface being polished of a workpiece and a polishing cloth is so small that sufficient polishing speed cannot be achieved for polishing the workpiece. According to the present invention, the processing surface which makes circulatory translational motion can be used because no large processing speed is required by the processing unit such as a cleaning unit.
In case that a surface of an abrading plate is used as a polishing surface or a processing surface, such an apparatus can satisfy a wide range of polishing needs, from rough grinding to finish polishing, by choosing an abrasive grain size, a method of supplying the polishing solution and a rotational speed to suit each work. That is, to perform rough polishing, abrading surface is made coarser and a relatively high speed and high pressing pressure are used. On the other hand, to perform finish polishing, abrading surface is made finer and a relatively low speed and low pressing pressure are applied. Removal of micro-particles adhering to the workpiece surface may also be performed during the finish polishing by using a solution appropriate to the purpose. Specifically, for rough polishing, abrasive grains are used while for finish polishing, deionized water and solutions may be used. Abrading grains are normally not used in finish polishing, but if they are needed, a small amount of ultra-fine micro-grains is used.
The circulatory translational motion is defined as xe2x80x9ca relative motion between a first surface and a second surface facing the first surface and a non-rotational motion which causes every point on the first surface to describe a substantially identical locus with respect to the second surface.xe2x80x9d The locus maybe a circle, an ellipse, a polygon, or any other regular shape. For a better polishing ability and mechanical reasons, the circulatory translational motion should preferably be made along a circular path. The circulatory translational motion along the circular path allows the confronting surfaces to move relatively to each other uniformly in different areas thereof.
The circulatory translational motion of this invention has the same meaning as orbital motion.
In preferred aspects, the polishing surface may comprise a surface of a polishing cloth or a surface of an abrading plate. The polishing surface may rotate about its rotating axis or make relative translational motion relative to the workpiece. The translational motion of the polishing surface may be provided only by moving the polishing surface.
The processing may comprise polishing of the polished surface of the workpiece or cleaning of the polished surface of the workpiece. The processing surface may comprise a surface of a polishing cloth or a surface of a wiping cloth or a surface of an abrading plate. The relative translational motion of the processing surface may be provided only by moving said processing surface.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.